1. Field of the Invention
The invention relates to a cooling system for cooling a metal strip which is moving substantially horizontally along a roller conveyor e.g. in a steel-making plant. The cooling system comprises water boxes located between successive rollers of the conveyor, each water box having upwardly directed outlet ducts extending parallel to each other and at uniform spacing. The invention also relates to a water box for use in such a cooling system.
2. Description of the Prior Art
One cooling system as described above is known from Dutch patent no. 145782 in which FIG. 3 shows that the outlet ducts of the water box run with divergence from the vertical. However, since this publication does not indicate the direction in which the strip moves, the significance of the slant of the outlet duct is not clear. An article written by employees of the patentee in "Iron and Steel Engineer", page 84, March 1971, FIG. 6 shows that the slant of the outlet ducts is intended to be in the direction of movement of the metallic strip, so that water emerging also has a component of motion in the direction of movement of the strip. This direction of the slant of the outlet ducts is also found in all the installations actually built by the patentee and its successors in title, as well as in drawings and reports of the patentee only available internally.
The desirability of such a slant was accepted on the grounds of the cooling effect which it can give to the succeeding roller in the roller conveyor, and furthermore because it was thought that the impulse o the water jets directed in the direction of movement of the strip achieved maximum effect for intensive contact of fluid and strip surface.
However, new understanding has led to another configuration of the cooling system. It has been found that the movement of the fluid in the restricted space between rollers, water box and moving strip is extremely complex, partly because of the high velocity of rollers and strip, and it is also possible that the great differences in temperature between strip and water jet may play a role. It has been discovered that there exists water film which is not easily penetrable and has an inconsistent thickness on the under-face of the strip. This film is not easily controllable and leads to a likewise inconsistent and consequently undesirable or uncontrollable cooling effect.
It must be assumed that as a result of the complex water motion described, which at the same time partly causes atomization, the water film on the underside of the strip has a smaller thickness after the strip has left the preceding roller than when it runs onto the next roller.
This new understanding has given rise to the invention described below, and experiments have shown that, surprisingly, the new configuration of the outlet ducts leads to a better cooling effect.
Other prior art to be mentioned includes GB-A-1568483 in which water boxes have compressed air chambers for propelling the water as non-laminar jets. At the underside of the strip, the jets are inclined both forwardly and rearwardly with respect to the strip movement direction. No mention is made of the cooling of the rollers of the conveyor by the water from the jets.
JP-A-60-43434 discloses a cooling system for thick steel plate (not strip) having jets for directing cooling liquid onto both surfaces of the plate in the rearward direction. Gas jets prevent diffusion of the cooling liquid in the forward direction. FR-A-1471847 discloses another system for cooling steel plate or slab in which cooling fluid apertures are directed in both the forward and rearward directions. FR-A-2552448 shows in FIG. 16 a similar system, applicable to both plate and sheet.